1. Field of the Invention
The present invention relates to drain systems for use with floating roofs in storage tanks, and more particularly to oil tank floating roof drain systems utilizing rigid pipes and swivel joints.
2. History of the Prior Art
It is well known to store volatile liquids such as oil and other petroleum products in large tanks of generally cylindrical configuration and having a floating roof. The floating roof which is of generally circular configuration and which is of slightly smaller diameter than the inside of the tank floats on the oil or other liquid in the tank so as to move up and down within the tank as the level of the liquid within the tank varies. The floating roof which protects the oil from external contaminates is normally provided with one or more seals which seal the small area between the outer edge of the floating roof and the inner tank wall while at the same time permitting the roof to move up and down within the tank. The seals greatly minimize or eliminate the escape of fumes or vapors from inside the tank so as to protect the environment, while at the same time preventing rain water, dirt and other external elements from entering the oil stored in the tank. Examples of such seals are provided by U.S. Pat. No. 4,308,968, SECONDARY SEAL FOR TANK HAVING FLOATING ROOF, Roger W. Thiltgen et al, which patent issued Jan. 5, 1982 and is commonly assigned with the present application.
Most such floating roofs are configured so as to drain toward a low point at the center thereof. This tends to direct rain water away from the outer edge of the floating roof where it might eventually damage the seal and flow into the oil within the tank. The low point at the center of the roof has a drain leading to a collecting sump at the underside of the roof so that rain water which collects on the roof can be safely routed into the sump and through a drain system to the outside of the tank. The drain system which basically resides within the tank and is submerged within the oil may comprise any one of a number of different basic arrangements. Traditionally, floating roof drain systems were comprised of lengths of relatively rigid pipe joined together by swivel joints to provide a flexible arrangement which extended from the sump to a lower portion of the tank wall. At the lower portion of the tank wall, one of the lengths of rigid pipe exited the tank so as to drain the rain water collecting at the center of the roof to a location outside of the tank where it could be disposed of safely and properly. Another type of drain system which has come into widespread use in recent years utilizes a length of flexible pipe or conduit coupled between the collecting sump and the tank wall, and disposed within the oil or other liquid in the tank.
The drain systems for use with floating roofs must be capable of flexing through a substantial range of movement so as to accommodate vertical movements of the floating roof between an uppermost position at the top of the tank and a lowermost position in which the roof resides at a relatively small distance above the bottom of the tank. In addition, such drain systems must be capable of relatively leak-free operation in the face of a substantial pressure differential. The pressure of the oil or other liquid within the tank is usually substantially greater than the pressure within the drain system itself. The natural tendency is for the higher pressure oil to leak into and then escape through the drain system. Because of the relatively large size and resulting bulkiness of typical drain systems, they tend to encounter substantial stresses on various parts thereof, which stresses can damage or destroy the parts and render the drain system ineffective or useless. Because gravity is relied upon to drain the rain water from the floating roof through the drain system, the drain system must be such as to provide a positive gravity feed at all times and in the face of the varying levels of the floating roof.
In the case of drain systems of the rigid conduit, swivel joint type, the swivel joints are designed to rotate or pivot about a given axis. Typically, two lengths of curved pipe are made rotatable relative to each other by a bearing which is sealed. Despite the sealing of the bearings, the seals tend to fail or leak rather readily. This is due largely to the fact that practically all of the stress placed on the swivel joint occurs at the seal bearing where the relative motion takes place. It is also due to the fact that such swivel joints are normally designed for use in other environments in which the pressure within the joint is far greater than on the outside of the joint. Most such joints are not designed to withstand a greater external fluid pressure. Seal or bearing damage or failure may result in leakage of the oil from inside the tank into the drain system. It can also result in leakage of rain water and other contaminates into the oil within the tank. Repair or replacement of damaged swivel joints is time consuming and expensive and frequently requires that the tank be completely drained so that the drain system can be worked on from within the tank.
It was in the face of such problems that the flexible conduit type of drain system was developed. Such systems typically utilize a single length of flexible pipe or conduit which extends between the collecting sump at the underside of the center of the floating roof and a lower portion of the tank wall. The flexible conduit flexes or bends as necessary to accommodate the upward and downward movements of the floating roof while at the same time providing a drain path for rain water or other material collected on the roof. The flexible conduit is normally made flexible along the entire length thereof so that no single portion thereof is subjected to substantially greater stress than other portions thereof. The need for sealed bearings within joints is thereby eliminated.
However, flexible conduit drain systems are not without problems of their own. For one thing, it is relatively expensive to design and build a flexible conduit which can maintain an effective seal between the inside and the outside thereof in the face of the substantially greater external pressure differential and the highly aromatic nature of oil, gasoline and other petroleum products. Another problem resides in the tendency of the flexible conduit to float. As a result the flexible conduit frequently forms loops or at least high points therein. This interferes with the positive gravitation operation of the drain system, and may require that weights or ballast be added to portions of the flexible conduit to counteract the tendency for the conduit to float.
The relative merits of the rigid conduit, swivel joint type of drain system and the flexible conduit type of drain system are discussed in an article by Ray E. Decker entitled "Careful Design Remains Key To Good Floating-Roof Drains" at pp. 107-110 of the June 1, 1981 issue of OIL AND GAS JOURNAL. The article discusses both types of drain systems before concluding that the flexible conduit type of system holds the most promise because of the problems inherent in the use of the swivel joints in the rigid conduit, swivel joint types of systems.
The Decker article points to the fact that most of the problems in the rigid conduit, swivel joint type systems reside in the swivel joints presently being used. The article also recognizes the problems inherent in the flexible conduit type systems, but feels that these problems can be more easily overcome than the basic problems characterizing the types of swivel joints currently used. The article thus points up the desirability of providing an improved swivel joint for use with drain systems of the rigid conduit, swivel joint type. The rigid conduit, swivel joint type of drain system which was among the first to be developed avoids some of the limitations inherent in flexible conduit and other types of systems and appears to be the clear choice for many applications if the problems present in the swivel joints can be minimized or eliminated.